1. Field of the Invention
The present invention relates to a process and apparatus for reducing chemical oxygen demand (COD) in water. More particularly, the present invention relates to a process and apparatus whereby polluted water is successively treated to remove organic and inorganic pollutants. Even more particularly, the present invention provides a process and apparatus therefor for treating polluted water utilizing both flocculation and catalytic oxidation.
II. Description of the Prior Art
As a result of growing concern for the purity of water resources and in response to growing governmental pressures to maintain the quality of these water resources, industry and other large-scale water consumers have been required to investigate and implement methods for reducing chemical pollutants contained in their effluent streams. The danger of chemical pollution in water is due, in part, to the ability of organic constituents to bind dissolved oxygen contained in the water. This binding, whether by chemical reaction or simple chemical interaction, prevents the utilization of dissolved oxygen by aquatic life. The effect of this binding is commonly referred to as chemical oxygen demand (COD).
Because of the importance of maintaining adequate levels of dissolved osygen in water streams, various governmental agencies, such as the U.S. Environmental Protection Agency, have set forth guidelines and test procedures for measuring the COD of effluent streams entering rivers and lakes. Industrial, municipal and other large scale users of water are required by law to maintain COD of effluent streams originating at their facilities at or below mandated levels or face legal penalties.
As the amount of discharged pollutants has risen with increases in population and industrialization, various processes have been implemented to improve the quality of the water discharged, as effluent, as established by the mandated standards. Most of these methods involve waste water treatment to eliminate pollutants in the effluent streams, prior to discharge into sewers. Among the method proposed are (1) evaporation followed by incineration, (2) chemical treatment to render the organic constituents in the effluent harmless, (3) biological treatment and aeration of effluent collected in holding tanks and (4) oxidation of the chemical constituents under restrictive conditions. Each of these methods has drawbacks as outlined below.
The first method, evaporation followed by incineration, is impractical where large volumes of water containing minute percentages of organic pollutants are involved. The procedure is extremely slow and imposes exorbitant expenditures in both manpower time and dollars.
Chemical treatment, such as chlorination, ozonation, treatment with hydrogen peroxide, etc., can be dangerous and expensive and is effective only on specific chemical pollutants. Thus, chemical treatment is not a general solution to the entire waste disposal problem. Furthermore, such treatment generally depends upon accurate analysis of the materials present in the waste effluent to ascertain what procedures are required to initiate predetermined chemical reactions to produce desired reaction products. Where such treatment is feasible, it generally results in focusing attention on by-product recovery rather than on the water disposal problem itself. The resulting waste effluent frequently contains different, but equally harmful pollutants after treatment; in some cases, these resulting pollutants are more dangerous than the initial ones. As a practical matter, in order to be effective, any chemical treatment method employed requires testing by expensive processes to ascertain the precise nature of the contaminants.
One of the major types of purification of organically polluted waste effluent involves the use of biological processes. The biological processes use microorganisms, such as bacteria or protozoa, to decompose chemical pollutants. In the biological system, industrial waste water first receives conventional primary treatment after which it is retained for a number of hours in large aeration tanks containing microorganisms. During this process, air is constantly diffused through the contained water, to support the microorganisms, as they use available carbon in the organic matter as a source of food, thereby metabolizing the compounds contained in the water to carbon, hydrogen and water, simultaneously rendering these compounds harmless.
Biological processes are limited by the nature of the microorganisms themselves. Great care must be exercised to maintain an environment in which the microorganisms will function. The temperature of the waste water must be carefully regulated to prevent adverse effects at the range where the microorganisms are active. The waste water must be supplemented with appropriate nutrients and other additives which allow the microorganisms to function at the desired level of efficiency. Likewise, care must be taken that the waste water contains no bactericidal or bacteriostatic compounds, e.g. formaldehyde and hexachlorophene, which will cause bacterial activity to cease. Furthermore, specific strains of microorganisms are specifically suited only for the decomposition of specific chemicals. Thus, an industry employing a biological process must be able to provide a system which combines specific strains of microorganisms specifically developed to decompose the various organic pollutants present in that industry's effluent streams. This requirement renders biological systems impractical and exigent. New or different organic constituents which enter the waste water being processed must lend themselves to the desired effect of the existing microorganisms, or the existing biological treatment system must be modified to accommodate the change in the compounds introduced into the system. This requirement is unreliable, costly and demands considerable maintenance time.
Even at peak performance, the biological system present several drawbacks. The microorganisms have a short life span. The system must be constantly seeded with new microorganisms in order to assure proper function of the system. The microorganisms reproduce and die constantly, placing an additional demand on the system, which will eventually cause the entire biological system to fail if the dead microorganisms are not removed. The removed organic mass presents a serious solid disposal problem. The microorganisms form a bulky, malodorous, gelatinous mass. Disposal routinely involves costly shipment from the location of the water treatment facility to disposal facilities where the organic mass is incinerated or buried in environmentally safe landfills.
While the biological system does provide biodegradation causing reduction in the COD of waste water, it is time-consuming, unpredictable and, in many cases, only marginally efficient. Because of these drawbacks, interest has been directed to other processes which would reduce COD without requiring microorganisms or potentially harmful chemical reactions. Most prominent among these is oxidative treatment.
The basic process for oxidative treatment is set forth in U.S. Pat. No. 2,665,249 to Zimmerman and U.S. Pat. No. 3,133,016 to Stine. Both Stine and Zimmerman teach the reduction of the total pollutant content of waste water by reacting the organic-containing water with oxygen at an elevated temperature. The Zimmerman process requires a reaction temperature of at least 450.degree. F. to assure decomposition of the organic constituents. Stine teaches the use of superatmospheric pressures and temperatures ranging from 200.degree. to 600.degree. F. However, neither process provides for variances in the levels of organic pollutants present in the water. Thus, the treatment plant operator has no effective method for controlling the quality of the purified water. Also, neither process addresses the problem that many of the pollutants present in effluent streams are not readily oxidizable in their native state, such as short-chain organic compounds which are solubilized by long-chain constituents. Thus, while the Zimmerman process may result in a reduction of the overall COD in the effluent stream, many toxic components can remain in the treated water and eventually be dispensed into lakes, waterways and streams.
Processes employing various catalysts have been proposed for the oxidative decomposition of organic components contained in waste water. Examples of such processes are those found in U.S. Pat. Nos. 3,804,756 to Callahan; 3,823,088 to Box, Jr. and 2,690,425 to Moses. Each of the processes disclosed requires elevated temperatures to maintain the reaction process to completion. Catalysts such as platinum, palladium, zinc, copper and other components can be used to lower the ignition or decomposition energy of the entrained organics. Ideally, in each process the energy released, as heat, upon the decomposition of certain entrained organic components is used to initiate further breakdown. However, in actual operation, external energy must continually be added to the system to continue breakdown. Thus, with certain compounds, temperatures as high as 1000.degree. F. may be required to initiate any appreciable breakdown of the entrained organic. Such temperatures require that the purification apparatus include high-pressure equipment and extensive supplemental heat transfer systems, since steam does not provide an effective means of transporting insoluble and soluble non-combustible salts and ash away from the reaction zone. Thus, systems, such as are disclosed in U.S. Pat. Nos. 2,944,396 to Burton; 4,294,706 to Kakihara and 4,141,828 to Okada, employ elevated pressures to maintain a portion of the waste stream in the aqueous phase to prevent build-up of residue in the reaction zone.
The above-discussed catalytic oxidation systems are concerned with haphazard decomposition of COD as a function of the total deleterious chemical constituents contained in water. A reduction in the COD level can occur when any of the chemical constituents are decomposed. It has been found that some water-soluble chemicals contained in waste water act as a solubilizing agent for normally insoluble constituents to the extent that the normally insoluble components cannot be removed by flocculation. The processes for COD reduction known in the prior art do not provide a systematic method for total breakdown of all chemical constituents together with a decrease in the associated COD, without employing elevated temperature and pressure. Thus, if the chemical contents of waste water can be systematically eliminated, the system can be more effective, more reliable and less expensive.
Furthermore, the methods for reducing COD in water previously developed do not provide an apparatus for effecting water purification catalytically in which the catalyst in one reactor can be regenerated while the purification process continues uninterrupted in others.
Thus, it would be desirable to provide a process for reducing chemical oxygen demand in waste water to levels at or below those mandated by the federal government.
It is also desirable that the process be carried out at ambient temperature and pressure.
It is also desirable that the process yield no undesirable by-products.
It would be desirable to provide a means for protecting any solid catalysts required for the process from excessive contamination.
It would also be desirable to provide a process for reducing COD in water in which the catalyst(s) and adsorptive material are present in a plurality of fixed beds and which can be regenerated in situ.
It is also desirable that the process include a means of isolating and regenerating individual reactors containing catalyst while the system remains in operation.